1. Field of the Invention
The present invention relates to a multilayered printed circuit board and a manufacturing method therefor, and more particularly to a method of manufacturing a multilayered printed circuit board with which an opening is formed by using laser beam and by forming a plated film in the opening to form a via hole, an opening in a solder-resist layer and an opening for forming a through hole. The present invention relates to the structure of a substrate which is a core.
2. Background Art
A method of manufacturing a multilayered printed circuit board has been suggested in Japanese Patent Laid-Open No. 9-130038 which uses a so-called conformal mask such that a conductive layer is formed on the surface of an insulating resin layer. Moreover, an opening is formed in a portion of the conductive layer by etching, followed by irradiating the opening with a laser beam to form an opening in the insulating resin layer.
The foregoing technique, which uses a thick copper film having a thickness of 12 mm to 18 mm as the conformal mask, encounters a large thermal conductivity, causing heat to easily be dispersed. Hence it follows that a high output laser beam or a pulse-shape laser beam must be applied a multiple of times. Therefore, when the opening is formed in the insulating resin layer, undercut occurs with which the opening is widened in the lateral. If a via hole is formed by applying an electrolytic copper plated film and an electroless copper plated film to the opening, separation of the electrolytic copper plated film and the electroless copper plated film easily occurs. As a result, reliability in the connection has been unsatisfactory.
To a worse extent, the foregoing technique cannot form a conductive circuit having fine pitch. In the manufacturing process, the electroless copper plated film (0.1 μm to 5 μm) and the copper foil (12 μm to 18 μm) under the resist must be removed after the electrolytic copper plated film has been performed. Therefore, the width of the conductive circuit cannot be reduced.
Since the thick copper foil is employed as the conformal mask, a via hole having a small diameter cannot be formed. In the manufacturing process, the electroless copper plated film (2 μm) and the copper foil (12 μm to 18 μm) under the resist must be removed, the diameter of the via hole cannot easily be reduced.
To solve the foregoing problems, an object of the present invention is to suggest a multilayered printed circuit board which is capable of preventing occurrence of undercut if a conformal mask is employed.
A method disclosed in Japanese Patent Publication No. 4-3676 and using a “conformal mask” has the steps of previously forming a metal layer on an insulating resin layer; etching and removing the metal layer in the portion in which a via hole will be formed; and irradiating the opening with a laser beam so that only the insulating resin layer exposed through the opening is removed. The foregoing technique, which is capable of forming a plurality of via holes in the insulating resin layer, exhibits satisfactory productivity.
However, studies performed by the inventor of the present invention have resulted in resin which is left in the opening for forming the via hole, causing the residual resin to expand and unsatisfactorily move the via hole in the upward direction. Thus, there arises a problem in that the upper and lower layers are electrically insulated from each other.
Another problem has been detected as a result of the studies performed by the inventor of the present invention in that the resin in the periphery of the opening is raised excessively and, thus, the via hole is disconnected.
A still further object of the present invention is to obtain a multilayered printed circuit board having furthermore improved reliability in the connection in the via hole portion.
On the other hand, a multilayer forming technique which employs so-called RCC (RESIN COATED COPPER: Copper film having resin) as the built-up multilayered printed circuit board has been suggested. The foregoing technique has the steps of laminating RCC on a circuit substrate; etching the copper foil to form a through hole in a portion in which the via hole will be formed; irradiating the resin layer in the through hole portion with a laser beam to remove the resin layer opening as to form an opening; and filling the opening with plating so that the via hole is formed.
Another technique has been developed as disclosed in Japanese Patent Laid-Open No. 9-36551 with which one-side circuit substrates each having a through hole filled with a conductive substance are laminated through adhesive layers so that a multilayered structure is formed.
The foregoing multilayered printed circuit board is subjected to a process for coarsening the surface of the lower conductive circuit to maintain the adhesiveness between the surface of the lower conductive circuit and the interlayer insulating resin layer.
The coarsening method is exemplified by a method (hereinafter called a “Cu—Ni—P plating method”) of covering and coarsening the surface of the conductive circuit with a needle shape or porous plating layer made of a Cu—Ni—P alloy; a coarsening method (hereinafter called a “blackening and reducing method”) with which the surface of the conductive circuit is blackened (oxidized) and reduced; a coarsening method (hereinafter called a “soft etching method”) which uses mixed solution of peroxide and sulfuric acid to soft-etch the surface of the conductive circuit; and a coarsening method (hereinafter called a “scratching method”) with which the surface of the conductive circuit is scratched with a sandpaper or the like.
However, if the conductive circuit is coarsened by the Cu—Ni—P plating method of the blackening and reducing method, followed by forming an interlayer insulating resin layer, and followed by applying a laser beam to form an opening for forming the via hole in the interlayer insulating resin layer, the coarsened surface of the conductive circuit is undesirably removed and flattened owing to the irradiation with the laser beam. Thus, there arises a problem in that the adhesiveness with the via hole formed above the coarsened surface becomes defective.
The reason for this lies in that the coarsened surface formed by the above-mentioned process is colored and, thus, the colored surface undesirably absorbs the laser beam.
When the coarsened surface has been provided for the conductive circuit by the soft etching method or the scratching method, the coarsened surface does not absorb the laser beam. Since the coarsened surface has not sufficiently been coarsened, there arises a problem in that satisfactory adhesiveness cannot be realized between the conductive circuit and the interlayer insulating resin layer.
To solve the above-mentioned problems experienced with the conventional technique, a still further object of the present invention is to provide a multilayered printed circuit board and a manufacturing method each of which is able to realize satisfactory adhesiveness with the interlayer insulating resin layer which is formed on the conductive circuit, with which flattening of the coarsened surface of the surface of the conductive circuit can be prevented when a laser beam is applied to form the via hole in the interlayer insulating resin layer and which has a via hole (conductive circuit) having satisfactory adhesiveness.
With the conventional technique, the via hole is formed by drilling a through hole in the interlayer insulating resin layer and by disposing a metal film in the through hole. Hitherto, the through hole has been formed by employing photosensitive resin to constitute the interlayer insulating resin layer and by exposing a position corresponding to the through hole through a mask on which a black circle has been drawn to sensitize the interlayer insulating resin layer so as to dissolve the non-sensitized portion corresponding to the position of the black circle.
The foregoing photolithography method, however, encounters a limitation of the smallest diameter of the through hole, the limitation being a diameter of about 80 μm. Therefore, the foregoing method cannot meet a requirement for raising the density of the multilayered printed circuit board. Therefore, the inventor of the present invention has come up with an idea that the through hole is formed by using a laser beam and performed experiments. As a result, a through hole having a diameter not larger than 80 μm can be formed.
However, a fact has been detected that the reliability of the via hole is unsatisfactory when the via hole has been formed by using the through hole having a diameter not larger than 80 μm. The cause of the foregoing fact has been studied, thus resulting in a problem of insufficient adhesiveness between the through hole and the electroless copper plated film. That is, the via hole is formed by depositing the electroless copper plated film. Satisfactory adhesiveness cannot be realized between the through hole formed by using a laser beam and having a small diameter and the electroless copper plated film. As a result, separation of the electroless copper plated film from the through hole causes disconnection to occur.
On the other hand, the conventional photolithography technique has been performed such that the through hole is formed by performing exposure and development. Therefore, only photosensitive materials are permitted to be used. Hence it follows that the performance required for the multilayered printed circuit board cannot sometimes be satisfied.
The conventional multilayered printed circuit board suffers from unsatisfactory reliability of solder bumps. The cause has been studied, thus resulting in the insufficient adhesiveness between the through hole and the metal film. That is, the solder bump is formed by enclosing solder in a portion on a nickel plated film deposited on the surface of the conductive circuit below the opening. The adhesiveness between the conductive circuit and the nickel plated film has been unsatisfactory, causing the nickel plated film to be separated. It leads to a fact that disconnection of the solder bump occurs.
To solve the above-mentioned problems, a still further object of the present invention is to provide a multilayered printed circuit board and a manufacturing method therefor each of which is capable of forming a via hole exhibiting satisfactory reliability and having a small diameter.
To solve the above-mentioned problems, a still further object of the present invention is to provide a multilayered printed circuit board and a manufacturing method therefor each of which permits selection of high-performance material for the solder resist.
A still further object of the present invention is to provide a multilayered printed circuit board and a manufacturing method therefor each of which is capable of forming a reliable solder bump.
On the other hand, the through hole provided for a core substrate must precisely be formed. Thus, a through hole having a diameter smaller than 100 μm cannot easily be formed by drilling. Therefore, the through hole is formed in a copper-plated laminated board by using a laser beam.
An optimum laser beam is a carbon dioxide laser because of a low cost from the viewpoint of industrial production. However, the carbon dioxide laser is undesirably reflected by the surface of the copper foil. Therefore, it is technical common sense that the through hole cannot be formed directly in the copper-plated laminated board by the laser machining. Thus, a technique has been disclosed in Japanese Patent Laid-Open No. S61-99596 with which the surface of the copper foil of the copper-plated laminated board is subjected to a blackening process (an oxidizing process), followed by applying a laser beam.
The foregoing technique, however, requires the blackening process to be performed first, causing a problem to arise in that a long manufacturing process is required.
As a result of energetic studies performed by the inventors of the present invention, an unexpected fact has been found that reduction in the thickness of the copper foil enables an opening to be formed in the copper foil in spite of the reflection by the surface.
To solve the above-mentioned problems, a still further object of the present invention is to realize a technique which is capable of directly forming an opening in a copper-plated laminated board and provide a substrate having the through hole formed by the foregoing method and a multilayered printed circuit board.
Since a high density and multilayered structure has been required in recent years, a built-up multilayered printed circuit board has attracted attention. The multilayered circuit board is a multilayered circuit board having a core substrate on which conductive circuits and interlayer insulating resin layers are alternately formed. The conductive circuits in the layers are connected to one another through a via hole.
As the core substrate of the foregoing built-up multilayered printed circuit board, a glass epoxy resin substrate of FR-4 grade has been employed.
However, the glass epoxy resin substrate of the FR-4 grade encounters a problem in that the insulation resistance between the through holes is reduced in a HAST test and a steam test. Another problem arises in that the resistance of the through hole chain was excessively changed as a result of the heat cycle test. That is, the reliability realized after use for a long time has been unsatisfactory.
To solve the above-mentioned problem, a core substrate which employs BT (Bis maleimide-Triazine) resin has been suggested. The foregoing substrate is, however, a costly substrate.
Inventors of the present invention have considered probability of preventing reduction in the insulation resistance between the through holes and change in the resistance of the conductive circuit for connecting the through holes to each other by using low-cost resin such as epoxy resin in place of the costly resin, such as the BT resin. As a result, an unexpected fact was found that the foregoing problem is caused from the Tg point of the resin.
According to the present invention, a low-cost multilayered printed circuit board is suggested which is free from reduction in the insulation resistance between through holes as a result of the HAST test and the steam test and change in the resistance of the conductive circuit for connecting the through holes to each other as a result of a heat cycle test.
The method of manufacturing the printed circuit board is broadly divided into a subtractive method (a Subtractive Process) and an additive method (an Additive Process). The subtractive method is also called an etching method which is characterized by chemically corroding the surface copper foil of a copper-plated laminated board. A method of manufacturing a printed circuit board (a double-side board) by the subtractive process will now briefly be described.
Initially, a copper-plated laminated board is prepared which incorporates an insulating substrate having two sides each of which is coated with copper foil having tens of μm. Then, an opening for forming a through hole is formed at a predetermined position of the copper-plated laminated board by drilling or the like. If a drilling step is performed, smears occurs in the opening for forming the through hole, desmear solution is used to process the copper-plated laminated board to dissolve and remove the smears. After the desmear process has been completed, electroless copper plating of the overall ground layer constituted by copper foil and the inner surface of an opening for forming the through hole is performed so that a thin plated layer is formed. After the foregoing plating process has been completed, a mask is formed on the thin plated layer. Then, thick plated layers are formed in the portions exposed through openings of the mask by electrolytic copper plating. After the foregoing plating process has been completed, the mask is separated. Then, etching is performed in a state in which an etching resist has been formed on the thick plated layer by solder plating or the like. The etching process is so performed as to remove the thin plated layer and the ground layer so as to divide the conductive pattern. Finally, the etching resist is separated so that a required printed circuit board is manufactured.
The foregoing method, however, is impossible to accurately form a fine pattern having a satisfactory shape. The characteristics of the etching causes a so-called divergent conductive pattern having a bottom which is longer than the top to easily be formed. Therefore, a pattern cannot easily be formed in a portion (for example, a bonding pad portion) which must have a fine and precise structure.
In view of the foregoing, a still further object of the present invention is to provide a method of manufacturing a printed circuit board with which a fine conductive pattern having a satisfactory shape can be formed.